1. Field of the Invention
The present invention generally relates to a cryogenic refrigerator which generates cold (a cold thermal energy causing an ultracold temperature) by generating Simon expansion using a high-pressure refrigerant gas supplied from a compression device.
2. Description of the Related Art
An example cryogenic refrigerator is disclosed in Patent Document 1. In this technique, a rotary motion of a motor is converted to a reciprocal motion by a scotch yoke mechanism to move a displacer.
In this cryogenic refrigerator, while reciprocating the displacer inside the cylinder and opening or closing a valve, a refrigerant gas inside an expansion space formed by the cylinder and the displacer is expanded to thereby generate cold. By transferring cold to a cooling stage positioned on a side of an outer periphery side on the expansion space, an object to be cooled can be frozen.
In the cryogenic refrigerator, provided that a position of the displacer in which an expansion space on a low temperature side is the minimum is called an “upper dead end” and a position of the displacer in which an expansion space on a low temperature side is the maximum is called an “lower dead end”, the refrigerant gas is taken in or supplied to by the valve before the displacer reaches the upper dead end. The upper dead end and the lower dead end can be in a manner opposite to the above.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2011-17457